The present invention relates to methods and apparatus for the supply of gases having a low water content and a low nitrogen content. The invention has particular relevance to the supply of gases from pressurised cylinders.
U.S. Pat. No. 5,409,526 describes a built in purifier for a gas supply cylinder. The purifier comprises a body of adsorbent in contact with the pressurised gas content of the cylinder, the adsorbent normally being arranged in a flow path for gas leaving the cylinder. There are several circumstances in which a supply of very dry gas may be needed, one being for use in the electronics industry, for instance in semiconductor fabrication. Whilst a water removing purifier containing adsorbent may be included in a supply line to be connected to a gas cylinder, operation costs can be reduced by maximising the life time of such a purifier by minimising the amount of water contained in the gas to which it is exposed. In U.S. Pat. No. 5,409,526, the adsorbent suggested for use in the built in purifier was Type 4A zeolite. The built in purifier described in U.S. Pat. No. 5,409,526 has proved very successful.
However, we have now discovered that in certain instances gas supplied from cylinders in which the gas is supposed to be essentially nitrogen free actually contains more nitrogen than it should, even though the bulk supply from which the cylinder was filled was satisfactorily low in nitrogen.